// Definition for a binary tree node.
function TreeNode(val, left, right) {
  this.val = (val === undefined ? 0 : val)
  this.left = (left === undefined ? null : left)
  this.right = (right === undefined ? null : right)
}

function createTree(arr) {
  if (arr.length < 1)
    return null;
  let NodeArr = [];
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] == null) {
      NodeArr.push(null);
      continue;
    }
    let tempNode = new TreeNode(arr[i], null, null);
    NodeArr.push(tempNode);
  }
  for (let i = 0; i < arr.length; i++) {
    if (NodeArr[i] == null) continue;
    if (2 * i + 1 < arr.length)
      NodeArr[i].left = NodeArr[2 * i + 1];
    if (2 * i + 2 < arr.length)
      NodeArr[i].right = NodeArr[2 * i + 2];
  }
  return NodeArr[0];
}

function preShow(node) {
  if (node == null)
    return;
  console.log(node.val);
  preShow(node.left);
  preShow(node.right);
}
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var levelOrder = function (root) {
  root = createTree(root);
  if(root == null)
    return [];
  if((root.left == null && root.right == null))
    return [root.val];
  let result = [], queue = [root];
  while(queue.length) {
    let node = queue[0];
    queue.splice(0, 1);
    if(node.left) queue.push(node.left);
    if(node.right) queue.push(node.right);
    result.push(node.val);
  }
  return result;
};
let root = [3, 9, 20, null, null, 15, 7];
console.log(levelOrder(root));
root = [1];
console.log(levelOrder(root));
